In the mining industry, depletion of high-grade ore over the years has invariably resulted in development of methods to utilize ore containing impurities and lower concentrations of the desired mineral. Low-grade impure ore is concentrated and purified to meet commercial standards, through various processes collectively referred to in the industry as "beneficiation". An overriding concern, of course, is efficiency. Any such method must be cost effective and competitive with the recovery of naturally high-grade ores.
The mining and purification of iron ore exemplifies this wide-spread phenomenon. Nowadays, it is common for lower grade ore to contain only about 35% iron. Typically, in one common beneficiation process, hematite, magnetite, goethitite, or martite-type ore is finely ground to liberate undesirable mineral impurities referred to as "gangue". Gangue, as found in most iron ore deposits, is a siliceous material such as quartz, clay, etc. and will hereinafter be referred to as silica, the presence of which adversely affects steel quality and the amount of slag bi-product generated in its manufacture.
After being ground, the ore or a concentrate thereof is then mixed with water to form a pulp, which is transferred to a large flotation cell. Air is introduced into and passed through the pulp. A frothing agent, usually a low molecular weight alcohol may be used. The froth formed is skimmed off or allowed to overflow. Undesirable silica floats away with the froth, leaving a more pure ore concentration for further processing into pellets or other suitable material.
In carrying out the flotation step, a collector agent capable of silica chelation is added to the pulp. Typically, ether or fatty amines and diamines are used to float or remove silica from iron ore. Common ether amines used include octyl/decyloxypropylamine available from Exxon Chemical Company under the designation "PA-1214". An additional ether amine which is commonly used to float silica is an ether amine partially neutralized with acetic acid. This ether amine is N-isotridecyloxypropyl-1,3-diaminopropane, commonly available from Exxon Chemical Company under the designation "DA-17". Such ether amines, called collectors, are surface active substances in which the main components are cationic.
In addition to silica, some iron ore bodies also contain low levels of phosphorous containing minerals such as various calcium phosphates, etc. which unless removed along with the silica during the flotation process becomes concentrated along with the iron in the ore. Apatite is a general term used to describe these phosphorous containing minerals. Such phosphorous containing compounds are undesirable in iron ore pellets. Phosphorous in steel makes it difficult to form into shapes, therefore, to meet quality standards it is necessary to reduce the phosphorous level in iron ore pellets. A commonly acceptable level is approximately 0.030-0.035% phosphate.
There have been numerous attempts to remove the phosphorous containing compounds at the iron ore mines via flotation. It is advantageous to remove apatite at the iron mines prior to forming the iron ore pellets to provide more pure pellets ready for further processing, rather than requiring further purification. Unfortunately, attempts to remove apatite at the mines has not been easily accomplished. Typically, apatite collectors are anionic in nature and therefore not compatible with cationic silica collectors. These incompatibility problems affect the apatite collecting ability and/or the silica collecting ability. In order to remove both the silica and apatite from the ore, a two-stage flotation is generally required. Several disadvantages are found in using a two-stage flotation. Mainly these disadvantages revolve around higher capital costs and potential greater iron loss due to the additional processing, separate water processing streams to eliminate cross contamination of the anionic and cationic flotation steps and duplicate flotation equipment. Additionally, added time and increased labor add to overall cost.
In summary, a considerable number of drawbacks and problems exist in the art relating to the removal of silica and phosphorous in ore flotation processes. There is a need for improved apatite collectors that are compatible with common silica collectors and can be used in a coflotation process.